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# Copyright 2018 Nordic Semiconductor ASA
# Copyright 2017 Linaro Limited
# Copyright 2019 Arm Limited
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""
Image signing and management.
"""
from . import version as versmod
from enum import Enum
from intelhex import IntelHex
import hashlib
import struct
import os.path
from cryptography.hazmat.primitives.asymmetric import padding
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes
from cryptography.exceptions import InvalidSignature
IMAGE_MAGIC = 0x96f3b83d
IMAGE_HEADER_SIZE = 32
BIN_EXT = "bin"
INTEL_HEX_EXT = "hex"
DEFAULT_MAX_SECTORS = 128
DEP_IMAGES_KEY = "images"
DEP_VERSIONS_KEY = "versions"
# Image header flags.
IMAGE_F = {
'PIC': 0x0000001,
'NON_BOOTABLE': 0x0000010,
'ENCRYPTED': 0x0000004,
}
TLV_VALUES = {
'KEYHASH': 0x01,
'SHA256': 0x10,
'RSA2048': 0x20,
'ECDSA224': 0x21,
'ECDSA256': 0x22,
'RSA3072': 0x23,
'ED25519': 0x24,
'ENCRSA2048': 0x30,
'ENCKW128': 0x31,
'DEPENDENCY': 0x40
}
TLV_SIZE = 4
TLV_INFO_SIZE = 4
TLV_INFO_MAGIC = 0x6907
boot_magic = bytes([
0x77, 0xc2, 0x95, 0xf3,
0x60, 0xd2, 0xef, 0x7f,
0x35, 0x52, 0x50, 0x0f,
0x2c, 0xb6, 0x79, 0x80, ])
STRUCT_ENDIAN_DICT = {
'little': '<',
'big': '>'
}
VerifyResult = Enum('VerifyResult',
"""
OK INVALID_MAGIC INVALID_TLV_INFO_MAGIC INVALID_HASH
INVALID_SIGNATURE
""")
class TLV():
def __init__(self, endian):
self.buf = bytearray()
self.endian = endian
def add(self, kind, payload):
"""Add a TLV record. Kind should be a string found in TLV_VALUES above."""
e = STRUCT_ENDIAN_DICT[self.endian]
buf = struct.pack(e + 'BBH', TLV_VALUES[kind], 0, len(payload))
self.buf += buf
self.buf += payload
def get(self):
e = STRUCT_ENDIAN_DICT[self.endian]
header = struct.pack(e + 'HH', TLV_INFO_MAGIC, TLV_INFO_SIZE + len(self.buf))
return header + bytes(self.buf)
class Image():
def __init__(self, version=None, header_size=IMAGE_HEADER_SIZE,
pad_header=False, pad=False, align=1, slot_size=0,
max_sectors=DEFAULT_MAX_SECTORS, overwrite_only=False,
endian="little"):
self.version = version or versmod.decode_version("0")
self.header_size = header_size
self.pad_header = pad_header
self.pad = pad
self.align = align
self.slot_size = slot_size
self.max_sectors = max_sectors
self.overwrite_only = overwrite_only
self.endian = endian
self.base_addr = None
self.payload = []
def __repr__(self):
return "<Image version={}, header_size={}, base_addr={}, \
align={}, slot_size={}, max_sectors={}, overwrite_only={}, \
endian={} format={}, payloadlen=0x{:x}>".format(
self.version,
self.header_size,
self.base_addr if self.base_addr is not None else "N/A",
self.align,
self.slot_size,
self.max_sectors,
self.overwrite_only,
self.endian,
self.__class__.__name__,
len(self.payload))
def load(self, path):
"""Load an image from a given file"""
ext = os.path.splitext(path)[1][1:].lower()
if ext == INTEL_HEX_EXT:
ih = IntelHex(path)
self.payload = ih.tobinarray()
self.base_addr = ih.minaddr()
else:
with open(path, 'rb') as f:
self.payload = f.read()
# Add the image header if needed.
if self.pad_header and self.header_size > 0:
if self.base_addr:
# Adjust base_addr for new header
self.base_addr -= self.header_size
self.payload = (b'\000' * self.header_size) + self.payload
self.check()
def save(self, path):
"""Save an image from a given file"""
if self.pad:
self.pad_to(self.slot_size)
ext = os.path.splitext(path)[1][1:].lower()
if ext == INTEL_HEX_EXT:
# input was in binary format, but HEX needs to know the base addr
if self.base_addr is None:
raise Exception("Input file does not provide a base address")
h = IntelHex()
h.frombytes(bytes=self.payload, offset=self.base_addr)
h.tofile(path, 'hex')
else:
with open(path, 'wb') as f:
f.write(self.payload)
def check(self):
"""Perform some sanity checking of the image."""
# If there is a header requested, make sure that the image
# starts with all zeros.
if self.header_size > 0:
if any(v != 0 for v in self.payload[0:self.header_size]):
raise Exception("Padding requested, but image does not start with zeros")
if self.slot_size > 0:
tsize = self._trailer_size(self.align, self.max_sectors,
self.overwrite_only)
padding = self.slot_size - (len(self.payload) + tsize)
if padding < 0:
msg = "Image size (0x{:x}) + trailer (0x{:x}) exceeds requested size 0x{:x}".format(
len(self.payload), tsize, self.slot_size)
raise Exception(msg)
def create(self, key, enckey, dependencies=None):
if dependencies is None:
dependencies_num = 0
protected_tlv_size = 0
else:
# Size of a Dependency TLV = Header ('BBH') + Payload('IBBHI')
# = 16 Bytes
dependencies_num = len(dependencies[DEP_IMAGES_KEY])
protected_tlv_size = (dependencies_num * 16) + TLV_INFO_SIZE
self.add_header(enckey, protected_tlv_size)
tlv = TLV(self.endian)
if protected_tlv_size != 0:
for i in range(dependencies_num):
e = STRUCT_ENDIAN_DICT[self.endian]
payload = struct.pack(
e + 'B3x'+'BBHI',
int(dependencies[DEP_IMAGES_KEY][i]),
dependencies[DEP_VERSIONS_KEY][i].major,
dependencies[DEP_VERSIONS_KEY][i].minor,
dependencies[DEP_VERSIONS_KEY][i].revision,
dependencies[DEP_VERSIONS_KEY][i].build
)
tlv.add('DEPENDENCY', payload)
# Full TLV size needs to be calculated in advance, because the
# header will be protected as well
tlv_header_size = 4
payload_digest_size = 32
keyhash_size = 32
cipherkey_size = 32
full_size = TLV_INFO_SIZE + len(tlv.buf) + tlv_header_size \
+ payload_digest_size
if key is not None:
full_size += tlv_header_size + keyhash_size \
+ tlv_header_size + key.sig_len()
if enckey is not None:
full_size += tlv_header_size + cipherkey_size
tlv_header = struct.pack(e + 'HH', TLV_INFO_MAGIC, full_size)
self.payload += tlv_header + bytes(tlv.buf)
# Note that ecdsa wants to do the hashing itself, which means
# we get to hash it twice.
sha = hashlib.sha256()
sha.update(self.payload)
digest = sha.digest()
tlv.add('SHA256', digest)
if key is not None:
pub = key.get_public_bytes()
sha = hashlib.sha256()
sha.update(pub)
pubbytes = sha.digest()
tlv.add('KEYHASH', pubbytes)
# `sign` expects the full image payload (sha256 done internally),
# while `sign_digest` expects only the digest of the payload
if hasattr(key, 'sign'):
sig = key.sign(bytes(self.payload))
else:
sig = key.sign_digest(digest)
tlv.add(key.sig_tlv(), sig)
if enckey is not None:
plainkey = os.urandom(16)
cipherkey = enckey._get_public().encrypt(
plainkey, padding.OAEP(
mgf=padding.MGF1(algorithm=hashes.SHA256()),
algorithm=hashes.SHA256(),
label=None))
tlv.add('ENCRSA2048', cipherkey)
nonce = bytes([0] * 16)
cipher = Cipher(algorithms.AES(plainkey), modes.CTR(nonce),
backend=default_backend())
encryptor = cipher.encryptor()
img = bytes(self.payload[self.header_size:])
self.payload[self.header_size:] = encryptor.update(img) + \
encryptor.finalize()
self.payload += tlv.get()[protected_tlv_size:]
def add_header(self, enckey, protected_tlv_size):
"""Install the image header."""
flags = 0
if enckey is not None:
flags |= IMAGE_F['ENCRYPTED']
e = STRUCT_ENDIAN_DICT[self.endian]
fmt = (e +
# type ImageHdr struct {
'I' + # Magic uint32
'I' + # LoadAddr uint32
'H' + # HdrSz uint16
'H' + # PTLVSz uint16
'I' + # ImgSz uint32
'I' + # Flags uint32
'BBHI' + # Vers ImageVersion
'I' # Pad1 uint32
) # }
assert struct.calcsize(fmt) == IMAGE_HEADER_SIZE
header = struct.pack(fmt,
IMAGE_MAGIC,
0, # LoadAddr
self.header_size,
protected_tlv_size, # TLV Info header + Dependency TLVs
len(self.payload) - self.header_size, # ImageSz
flags, # Flags
self.version.major,
self.version.minor or 0,
self.version.revision or 0,
self.version.build or 0,
0) # Pad1
self.payload = bytearray(self.payload)
self.payload[:len(header)] = header
def _trailer_size(self, write_size, max_sectors, overwrite_only):
# NOTE: should already be checked by the argument parser
if overwrite_only:
return 8 * 2 + 16
else:
if write_size not in set([1, 2, 4, 8]):
raise Exception("Invalid alignment: {}".format(write_size))
m = DEFAULT_MAX_SECTORS if max_sectors is None else max_sectors
return m * 3 * write_size + 8 * 2 + 16
def pad_to(self, size):
"""Pad the image to the given size, with the given flash alignment."""
tsize = self._trailer_size(self.align, self.max_sectors,
self.overwrite_only)
padding = size - (len(self.payload) + tsize)
pbytes = b'\xff' * padding
pbytes += b'\xff' * (tsize - len(boot_magic))
pbytes += boot_magic
self.payload += pbytes
@staticmethod
def verify(imgfile, key):
with open(imgfile, "rb") as f:
b = f.read()
magic, _, header_size, _, img_size = struct.unpack('IIHHI', b[:16])
if magic != IMAGE_MAGIC:
return VerifyResult.INVALID_MAGIC
tlv_info = b[header_size+img_size:header_size+img_size+TLV_INFO_SIZE]
magic, tlv_tot = struct.unpack('HH', tlv_info)
if magic != TLV_INFO_MAGIC:
return VerifyResult.INVALID_TLV_INFO_MAGIC
sha = hashlib.sha256()
sha.update(b[:header_size+img_size])
digest = sha.digest()
tlv_off = header_size + img_size
tlv_end = tlv_off + tlv_tot
tlv_off += TLV_INFO_SIZE # skip tlv info
while tlv_off < tlv_end:
tlv = b[tlv_off:tlv_off+TLV_SIZE]
tlv_type, _, tlv_len = struct.unpack('BBH', tlv)
if tlv_type == TLV_VALUES["SHA256"]:
off = tlv_off + TLV_SIZE
if digest == b[off:off+tlv_len]:
if key is None:
return VerifyResult.OK
else:
return VerifyResult.INVALID_HASH
elif key is not None and tlv_type == TLV_VALUES[key.sig_tlv()]:
off = tlv_off + TLV_SIZE
tlv_sig = b[off:off+tlv_len]
payload = b[:header_size+img_size]
try:
if hasattr(key, 'verify'):
key.verify(tlv_sig, payload)
else:
key.verify_digest(tlv_sig, digest)
return VerifyResult.OK
except InvalidSignature:
# continue to next TLV
pass
tlv_off += TLV_SIZE + tlv_len
return VerifyResult.INVALID_SIGNATURE